1. Field of the Invention
The present invention relates to a control apparatus for an inverter and, more particularly, to a novel control apparatus which is capable of improving the torque characteristic of an induction motor to enhance the driving performance thereof
2. Description of the Prior Art
The conventional control apparatus of such type known heretofore has a constitution comprising, as shown in FIG. 1, an AC power supply 1, an AC-DC converter 2 for converting an AC output of the power supply 1 to a direct current, a smoothing capacitor 3 connected in parallel with the output of the AC-DC converter 2, a DC-AC inverter 4 for producing a new alternating current by converting the direct current charged in the smoothing capacitor 3, an induction motor 5 functioning as a prime mover while being supplied with the AC power from the inverter 4, a frequency setting circuit 6, a cushion circuit 7 for enabling smooth follow-up with respect to the frequency set in the circuit 6, a voltage-frequency ratio circuit for producing a voltage command relative to the frequency, and a PWM generator 9 for determining the timing to switch the output of the inverter 4 in response to the voltage command produced from the voltage-frequency ratio circuit
In the constitution mentioned above, the following circuit operation is performed. When the frequency corresponding to a desired speed is set by the frequency setting circuit 6, the cushion circuit 7 functions in response to the output of the frequency setting circuit 6 and produces a frequency command for enabling follow-up with respect to the preset speed in such a manner as not to bring about any abrupt change in the current running speed of the inverter.
In this stage, the frequency command from the cushion circuit 7 is applied to the voltage-frequency ratio circuit 8, which then produces a voltage command in accordance with a predetermined voltage-frequency ratio.
The PWM generator 9 receives both the frequency command from the cushion circuit 7 and the voltage command from the voltage-frequency ratio circuit 8, and generates a PWM signal to turn on or off a switching element such as a thyristor in the inverter 4.
In the conventional inverter control apparatus of the above constitution, a slip S becomes great in the case of a heavy load when driving the induction motor 5 by the AC output of the inverter 4 and thereby decreases the impedance of a secondary circuit in the equivalent of the induction motor shown in FIG. 2, hence reducing the excitation winding voltage E. Consequently, the resultant magnetic flux is diminished to bring about a reduction in the output torque. Such a disadvantage is conspicuous particularly in a low frequency range.